1. Field of the Invention
This invention relates to a fused zirconia refractory material having high-temperature heat resistance and corrosion resistance and a method for producing the same, and more particularly to a fused zirconia refractory material of zirconia-calcia-yttria system and a method for producing the same.
2. Prior Art
Having properties such as a high melting point of about 2700.degree. C., high corrosion resistance and low thermal conductivity, zirconia(ZrO.sub.2) has been generally used for refractory materials.
On the other hand, zirconia is a multi-modification mineral which is monoclinic from room temperature to about 650.degree. C., tetragonal up to about 1100.degree. C. and cubic up to about 2700.degree. C. It shows an extreme thermal expansion and shrinkage of about 5% at the phase transition between monoclinic and tetragonal. Accordingly, when subjected to a heat cycle, i.e. repetition of heating and cooling, zirconia is cracked and is finally fructured.
In view of the above, zirconia for refractory materials has been stabilized to restrain the extreme thermal expansion and shrinkage by adding CaO or MgO as a stabilizer and applying a high-temperature heat treatment and thereby substituting a portion of Zr.sup.4+ with Ca.sup.2+ or Mg.sup.2+ in the form of a solid solution.
Said stabilized zirconia has different properties depending upon the stabilizer and its quantity used. For example, four variations have been known when CaO is used as a stabilizer.
1) Quantity=2.5 wt %
Stabilization rate is 60%. Grain strength is high. Thermal expansion rate is low. Hysteresis, that is, the difference between expansion rate and shrinkage factor during a heat cycle, is high.
2) Quantity=4 wt %
Stabilization rate is about 80%. Grain strength is rather smaller than 2.5 wt % CaO. Thermal expansion rate is higher. Hysteresis is low.
3) Quantity=7.5-12 wt %
Stabilization rate is 100%, that is a fully stabilized zirconia. Grain strength is comparatively lower and thermal expansion rate is higher in comparison with 2.5 and 4 wt % CaO. No hysteresis is observed.
4) Quantity=25-30 wt %
Consisting of a fully stabilized zirconia and CaZrO.sub.3. Grain strength is lower than 100% stabilized zirconia. No hysteresis is observed.
The development of the stabilized zirconia has contributed to the spread of a continuous casting method which can omit a reheating process of iron, and thereby the yield of products has been increased. Further, air contact can be minimized by the continuous casting method, and the quality of iron has been improved.
The stabilized zirconia refractory materials, however, are not free from a destabilization. As a result, it has not been satisfactory yet in a recent clean steel technology for producing a high tension steel. Thus the development of novel refractory materials have been strongly waited for.
CaO stabilized zirconia has been produced by adding CaO to zirconia and fusing and solidifying said mixture. In that process, there is the need to exclude carbide and dioxide produced in a fusing process and to diminish the strain in a crystal caused in cooling or in the solidifying process. Accordingly, an annealing operation was added later for oxidization and decarburization as well as removal of strain in order to restrain the occurrence of destabilization which causes the phase transition.
It is true that CaO stabilized zirconia is prevented from occurrence of destabilization in a high-temperature region of above about 1400.degree. C. However, when it is used for a tundish nozzle brick, long nozzle brick or submerged nozzle brick in a continuous casting method, CaO elutes upon contact with a molten steel, whereby destabilization is accelerated. Further, grain strength deteriorates remarkably at a high temperature so that destruction and dissolution of the nozzle are accelerated.
On the other hand, MgO stabilized zirconia has a high strength at a room temperature. But, in a wide region of below 1100.degree. C., periclase(MgO) and monoclinic zirconia coexist in a crystal structure, so that destabilization easily occurs at a heat cycle. In order to prevent the occurrence of destabilization, an annealing operation must be made for a long period of time, so that it is not suitable for industrial production. Furthermore, MgO mixed into a molten steel is hardly removed in a later process compared to CaO, and it forms a bar against the achievement of clean steel technology.
There have been developed various refractory materials and refractory brick in order to solve the above problems.
Japanese patent publication No. 50-30035 discloses a method for preventing the occurrence of destabilization of stabilized zirconia, which consists of processes comprising crushing a CaO or MgO stabilized zirconia into a lump, heating the crushed zirconia to higher than 1200.degree. C., cooling it to below 900.degree. C. and repeating said processes more than three times.
Japanese patent application laid open under No. 62-138327 discloses a method for obtaining a fused stabilized zirconia in an efficient manner without using an annealing operation for oxidation, by oxidizing fusing zirconia by O.sub.2 blown into an electric furnace and modifying a stabilization rate by controlling a cooling rate for solidification.
The above two methods, however, are not practical in industrial production because the processes take much time and are very complicated. Besides, when using either CaO or MgO independently, the above mentioned problems can not be solved.
An improvement of components has been also attempted.
Japanese patent publication No. 63-1274 discloses a heat-resistant structural material of Y.sub.2 O.sub.3 --ZrO.sub.2 system consisting of single crystal and multi crystal, which is obtained by solidifying fused materials including zirconia of 89 to 99 mol % and yttria of 1 to 11 mol %.
Japanese patent application laid open under No. 61-68372 discloses zirconia partially stabilized by yttria having high hardness and high toughness, which is obtained by cooling a uniform cubic crystal of high temperature which includes zirconia and yttria of 1-10 mol % and thereby forming a structure of rhombohedral crystal and/or tetragonal crystal.
These materials are heat-resistant structural materials which are obtained by solidifying fused materials. Although it can be conjectured that they are superior in grain strength, heat resistance and corrosion resistance, it is not practical from an economical point of view to use yttria of over 1 mol % (1.82 wt %, approximately 2 wt %) for producing refractory materials, because yttrial is expensive. Generally the addition of over 3 mol % (5.3 wt %) is required when yttria is used as a stabilizer. A large quantity of yttria which amounts to about 15 wt % is required for obtaining a fully stabilized zirconia. In case the yttria content is less than 2 wt %, stabilization effect is poor.
Another refractory material has been disclosed in the Japanese patent application laid open under No. 60-51663. Said application discloses fused cast refractories of thermal shock resistant zirconia including MgO of 1 to 5 wt % and CeO.sub.2 of 0.2 to 6 wt %. But the employment of MgO badly affects the stability and the destabilization as mentioned above is apt to be induced.
As mentioned above, conventional refractory materials are not well restrained from destabilization. There are also such problems that the corrosion resistance to a molten steel is low and the processes are time consuming, complicated and uneconomical. As a result they are not suitable for the clean steel technology.